Digital image sensors implement an array of photodiodes and electrical connections connecting the photodiodes to readout electronics onboard the image sensor. In a traditional frontside-illuminated image sensor, the electrical connections are placed closer than the photodiode array to the light-receiving surface of the image sensor, such that light from the scene must pass through a wiring layer (accommodating the electrical connections) before reaching the photodiode array. This prevents detection of a significant fraction of incoming light. Backside-illuminated image sensors solve this problem by flipping the silicon substrate and thinning the backside of the silicon substrate to allow for light to enter through the backside and be detected by the photodiode array without first having to pass through the wiring layer. As a result, backside-illuminated image sensors may achieve higher sensitivity than frontside-illuminated image sensors.
However, the architecture of backside-illuminated image sensors presents challenges. For example, the silicon substrate of a backside-illuminated image sensor is significantly thinner than that of a frontside-illuminated image sensor, which renders backside-illuminated image sensors more fragile than frontside-illuminated image sensors. Another challenge is increased crosstalk between adjacent photodiodes in the absence of a wiring layer to provide at least partial light blockage between light propagation paths associated with different photodiodes. In color-sensitive backside-illuminated image sensors, such optical crosstalk may lead to color mixing and thus reduced color clarity in captured images.